As the production process of a metallic foil, there are various processes according to the quality of the materials and the uses of the metallic foil being produced but a process of producing a metallic foil by rolling and a process of producing a metallic foil by electrolysis are typical.
Copper foils which are used for printing plate boards for electric circuits are almost always produced by electrolysis. This is because in the case of an electrolytic copper foil, even when an inexpensive scrap copper, etc., is used as the raw material, the purity of the metallic copper deposited is improved by the principle of the electrolytic refining, and also the copper foil having a uniform thickness over a wide area can be easily obtained.
Furthermore, the texture of a metal deposited by electrolysis is suitable for the purpose of forming an electric circuit and has the feature of easily applying etching having a large aspect ratio. Also, another reason for the production by electrolysis being typical is that the cost for the metallic foil produced is lower than that of a metallic foil produced by rolling.
As shown in FIG. 1 of the accompanying drawings as the cross sectional view of an example of a copper foil producing apparatus by electrolysis, an electric current is passed through a large electrode roller 4 the lower portion of which is immersed in an electrolyte 3 in an electrolytic tank 2 as a cathode and an insoluble anode 5 as a counter electrode and while continuously plating copper on the surface of the cathode roller by supplying an electrolyte from an electrolyte-supplying slit 6 of the anode 5, the metallic copper 7 deposited is continuously scraped from the surface of the cathode roller as a copper foil. This process has the features that the average thickness of the copper foil 8 obtained can be easily controlled by controlling the amount of the electric current supplied and also a thin copper foil can be easily formed.
The electrolytic copper foil being used widely is continuously produced initially by a process of using a lead alloy as an anode, electrolytically depositing copper on the surface of a drum-type cathode made of lead, and winding the deposited copper foil while scraping the copper foil from the surface of the cathode. However, although the lead alloy anode is said to be insoluble, the consuming speed of the anode is very high as about several mg/Ah. Also, with the consumption of the anode, lead is dissolved in a sulfuric acid acidic copper sulfate solution as the electrolyte but since the solubility of lead is less in sulfuric acid, lead exists in the electrolyte as the insoluble particles of copper sulfate. As the result-thereof, it frequently happens that the particles of lead sulfate is intermixed in the copper foil obtained by electrolysis as if the particles were dispersed in the foil, and give bad influences on the copper foil. In particular, this is a large problem in the production of a thin copper foil having a thickness of not thicker than 25 .mu.m.
In order to remove the lead component from the electrolyte, a process of adding strontium carbonate, etc., to the electrolyte to co-precipitating the lead component and separating the precipitates by filtration is carried out but since the foregoing process is not a process of preventing the dissolution itself of the lead alloy electrode, the contamination of the electrolyte caused by the successively occurring dissolution of the lead alloy electrode can not be prevented. Also, as the result of the dissolution of the anode, there appears a problem that the distance between the cathode and the anode can not be maintained at a constant distance for a long period of time, and also there occurs a problem that the electrolytic tank and the electrolyte must be frequently repaired.
Furthermore, for solving the problem of the dissolution of the lead alloy anode, an insoluble metal electrode formed by coating a substrate of a thin film-forming metal such as titanium, a titanium alloy, etc., with an electrode active material containing a platinum group metal or the metal oxide has been used as the anode as disclosed in U.S. Pat. No. 4,318,794. The consumption of the insoluble metal electrode by the dissolution of the electrode material is from 1 to 0.1 mg/kAh or lower, which is from about 1/1,000 to 1/10,000 of the consumption of a lead alloy, and thus substantially no contamination of the electrolyte and the metallic foil as the product by the dissolution of the electrode occurs.
Also, such an insoluble metal electrode is very stable and can be continuously used almost as it is for several thousands hours. Furthermore, the deterioration of the electrode is not, in many cases, the deterioration of the electrode active material but occurs by the formation of a passive oxide film between the electrode substrate and the electrode active material.